Advance advisory of poor gardening weather

I can't speak for far more Northern climes, but down here, a May frost is always an overnight thing, short lived and not very hard at all. A consecutive night or two at at most in any year. Anyone who wants to protect vulnerable plants, can do so with a bit of effort.
 
There are alternatives to heated environments. Well insulated cold frames and greenhouses ( particularly twin wall polycarbonate ) cope very well with a careful eye on the weather. Anyone can cobble together an insulated cold frame and put their vulnerable plants in it at the appropriate times. Double up the cover overnight if necessary ... can use anything, even a supported tarp over a cold frame, no light lost at night.
Far better to sow in plenty of time, rather than sowing too late and missing the harvest due to an as yet unknown Summer. And certainly better than buying in hugely expensive garden centre plants.

It's easy to keep things frost free, agreed. It's not easy to create a warm atmosphere for plant such as tomatoes etc, that need more than just protection from frost, and certainly not warm enough in a cold frame, weather it's double lined or not.
Not sure how effective a piece of fleece thrown over plants that have already been planted out in greenhouse borders will be this weekend. Insulating, however well done can still only help to contain whatever temp it already is.
If you have vulnerable plants such as squashes, tomatoes, chillies etc, I would strongly recommend not putting them in anything that's not heated if the forecasts for heavy frost come true. No matter how much fleece you have.

Totty

I have found through trial and error (rather than listening to others wiser than me!), that stress free gardening can be enjoyed in mid and southern areas, if the quick growing tenders such as courgette and runners are direct sown on the third week of May. Every single attempt at planting earlier has failed miserably. Even if you start off indoors you still have to muck about hardening off in the strong sunshine. Sow on the 20th May and you will have strong healthy crops.

Last year, I tried starting off courgette indoors. I tried hardening them off and the blighters got sunburn and died 

Every single attempt to sow beans direct outdoors has failed for me. They just don't come up at all. That is the main reason why I start indoors

My toms, chillies, aubergine and courgettes have been holidaying in the greenhouse (non heated) for the last two weeks ... Come Friday its a trip back and forth bringing them all in before the cold snap ... Its just not worth the risk. Pots will be earthed and covered with fleece.
question beans slowly germinating not quite through the soil in their pots... Will they be ok or shall they come in too? Current in a mini greenhouse in the greenhouse ...

Just as I go on holiday    Typical!

Everything has 2 choices........... it'll just have to be  a trip to the garden centre if they make the wrong one